I'm a White Sox fan and we're pretty notorious for not showing up, especially when the team is bad. Showing up to support our team no matter how bad they are is a value they might hold on the north side, but we don't hold it. This was especially true in the decade between the 1994 strike (when the White Sox were in first place and the Sox owner was leading the hardline owners in favor of the strike) and the 2005 World Series year, many fans proudly boycotted the team during at least part of that time. Right now the team is historically bad, and there's very cold April Chicago weather on top of that. The low attendance does not surprise me. Frankly, I'd rather go see a Winston-Salem Dash game than a Chicago White Sox game, so I could see the rebuild prospects who are getting all the hype and not the filler currently on the major league roster.
That said, I think MLB attendance is down across the board. I don't think you can blame the quality of particular teams or the weather in particular cities for the big trend. My long-term prediction for sports is that in person attendance will continue to decline as the cost of TVs goes down and the ease of online streaming goes up.